Organic electroluminescent devices such as organic EL lighting devices have recently attracted attention as eco-friendly green devices. The organic EL lighting devices are characterized by 1) lower power consumption than incandescent lamps; 2) thin profile and light weight; and 3) flexibility. The organic EL lighting devices are now being developed to achieve the features 2) and 3). In this respect, glass substrates conventionally used in flat panel displays (FPD) or the like cannot achieve the features 2) and 3).
In this regard, researches have been conducted on a substrate as a support (hereinafter, referred to as “supporting substrate”) for organic EL lighting devices, and ultra-thin glass plates, resin films, and metal foils have been proposed as candidate supporting substrates. The ultra-thin glass is superior in heat resistance, barrier performance, and optical transparency and has good flexibility, but is somewhat inferior in handling and has low thermal conductivity and high material cost. The resin film is superior in handling and flexibility and has low material cost and good optical transparency, but is inferior in heat resistance and barrier performance and has low thermal conductivity.
In contrast, the metal foil has excellent characteristics such as superior heat resistance, barrier performance, handling, and thermal conductivity, good flexibility, and low material cost, except for absence of optical transparency. In particular, a typical flexible glass or film has a significantly low thermal conductivity of 1 W/m° C. or lower, while a copper foil has a significantly high thermal conductivity of about 400 W/m° C.
Techniques for corrosion protection of metal foil used in electronic materials are known which involves application of organic nitrogen compounds on the metal foil for preventing surface oxidation and/or hydroxylation (e.g., refer to Patent Document 1: JPH08-158074A).
Patent Document 2 (JP2009-152113A) discloses formation of an organic layer on a surface of a metal substrate that is smoothed by polishing and/or plating, in order to achieve a light-emitting device including the metal substrate. Patent Document 3 (JP2008-243772A) discloses formation of an organic EL device on a smooth surface of a nickel plating layer that is formed on a metal substrate without polishing. In these techniques, however, the smoothing of the metal substrate is an important challenge for prevention of a short circuit between the electrodes. For addressing this challenge, Patent Document 4 (WO2011/152091) and Patent Document 5 (WO2011/152092) disclose metal foil having an ultra-smooth surface having an extremely low arithmetic mean roughness Ra of 10.0 nm or less that functions as both supporting substrates and electrodes. Such techniques have been established for forming an organic semiconductor layer directly on a metal foil electrode, enabling emission of light. Unfortunately this can achieve only about 80% of external quantum efficiency compared to an electrode formed on a glass substrate and therefore further improved luminous efficiency is desired.